Nine countries have called for an emergency meeting of the UN Security Council to discuss the reports: the UK, France, the US, Poland, Netherlands, Sweden, Kuwait, Peru and Ivory Coast. It is due to take place later.

The allegations have been denied by the Syrian government and Russia, with the latter separately calling for a Security Council meeting over "threats to international peace and security".

Syrian media accused anti-Assad fighters in Douma of making "chemical attack fabrications" and added that allegations of toxic gas use are an "unconvincing broken record".

Iran branded the allegations as a "conspiracy" against Mr Assad, and warned that any US military intervention would "certainly complicate the situation" in Syria and the wider region.

Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...

The country's official IRNA news agency said the alleged attack had been condemned by the foreign ministry, which said in a statement: "Such allegations and accusations by the Americans and certain Western countries signal a new conspiracy against the Syrian government and people, and a pretext for military action."

It continued: "With the Syrian army having the upper hand on the ground against the armed terrorists, it would not be rational for it to use chemical weapons."

Mr Trump had earlier tweeted: "Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...

"....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"

.@POTUS & I closely monitoring likely chemical attack in Syria. We condemn in the strongest possible terms the assault on innocent lives, including children. The Assad regime & its backers MUST END their barbaric behavior. As POTUS said, big price to pay for those responsible!

Foreign Secretary Boris Johnson described the allegations of a chlorine gas attack in the rebel holdout as "deeply disturbing" and called for an immediate investigation by the Organisation for the Prohibition of Chemical Weapons.

"It is truly horrific to think that many of the victims were reportedly families seeking refuge from airstrikes in underground shelters," he said.

"Despite Russia's promise in 2013 to ensure Syria would abandon all of its chemical weapons, international investigators mandated by the UN Security Council have found the Assad regime responsible for using poison gas in at least four separate attacks since 2014."

"These latest reports must urgently be investigated and the international community must respond."

Image:People rush to help a distressed young child after the alleged chemical attack in Douma

Labour leader Jeremy Corbyn said: "The horrific deaths and injuries in Douma point to a chemical attack which must be fully investigated by the UN and those responsible held to account.

"The need to restart real negotiations for peace and a political settlement in Syria could not be more urgent."

Image:Douma in eastern Ghouta has been dubbed 'hell on earth'

The EU has called on Russia and Iran as "supporters of the regime" to use their influence with Damascus to prevent a similar attack.

Last year, the UN was among those blaming Syrian government forces for a deadly sarin gas attack on the opposition-held village of Khan Sheikhun in which at least 100 people died.

That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base, which President Trump said was meant to "prevent and deter the spread and use of deadly weapons".

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Video:April 2017: Syrian TV shows 'missiles hitting airbase'

Responding to the latest allegations, the Russian foreign ministry warned: "The goal of these false speculations, which are not substantiated by any facts, is to cover up terrorists and irreconcilable radical opposition, which opposes political settlement, and to simultaneously try to justify potential external military strikes.

"It is necessary to once again caution that military intervention under false and fabricated pretexts in Syria, where the Russian servicemen stay at the request of the legitimate government, is absolutely unacceptable and may trigger the gravest consequences."

British MP Johnny Mercer, who has previously claimed it was "the biggest foreign policy mistake of the decade" not to take military action in Syria, tweeted: "Look at the consequences of inaction."